Method and device for requesting system information

ABSTRACT

Provided are a method for a user equipment (UE) to request system information in a wireless communication system and a device supporting the same. The method may include: transmitting a random access preamble for requesting system information to a base station (BS); receiving, from the BS, a random access response including only a random access preamble identifier (RAPID) corresponding to the transmitted random access preamble; and considering that a random access procedure is completed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.17/336,945, filed on Jun. 2, 2021, now allowed, which is a continuationof U.S. patent application Ser. No. 17/020,068, filed on Sep. 14, 2020,now U.S. Pat. No. 11,102,810, which is a continuation of U.S. patentapplication Ser. No. 16/820,137, filed on Mar. 16, 2020, now U.S. Pat.No. 10,827,520, which is a continuation of U.S. patent application Ser.No. 16/261,876, filed on Jan. 30, 2019, now U.S. Pat. No. 10,660,124,which is a continuation application of U.S. patent application Ser. No.16/064,968, filed on Jun. 21, 2018, now U.S. patent Ser. No. 10/834,752,which is the National Stage filing under 35 U.S.C. 371 of InternationalApplication No. PCT/KR2018/001308, filed on Jan. 31, 2018, which claimsthe benefit of U.S. Provisional Application No. 62/453,469 filed on Feb.1, 2017, the contents of which are all hereby incorporated by referenceherein in their entirety.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a wireless communication system and,more particularly, to a method in which a UE requests other systeminformation and a device supporting the same.

Related Art

In order to meet the demand for wireless data traffic soring since the4th generation (4G) communication system came to the market, there areongoing efforts to develop enhanced 5th generation (5G) communicationsystems or pre-5G communication systems. For the reasons, the 5Gcommunication system or pre-5G communication system is called the beyond4G network communication system or post long-term evolution (LTE)system.

System information refers to essential information for communicationbetween a terminal and a base station. In 3GPP LTE, the systeminformation is divided into an MIB (Master Information Block) and an SIB(System Information Block). The MIB is the most essential information.The SIB is subdivided into SIB-x forms according to its importance orcycle. The MIB is transmitted through a PBCH (Physical BroadcastChannel) which is a physical channel. The SIB is common controlinformation and is transmitted through a PDCCH differently from the MIB.

SUMMARY OF THE INVENTION

The number of system information blocks is continuously increasing, andradio resources are required to broadcast a system information block.Thus, as the number of system information blocks increases, the quantityof radio resources required to broadcast a system information block alsoinevitably increases. To transmit continuously increasing systeminformation to a user equipment (UE), it is necessary to propose amethod for requesting system information that efficiently utilizes radioresources.

According to an embodiment, there is provided a method for a UE torequest system information in a wireless communication system. Themethod may include: transmitting a random access preamble for requestingsystem information to a base station (BS); receiving, from the BS, arandom access response including only a random access preambleidentifier (RAPID) corresponding to the transmitted random accesspreamble; and considering that a random access procedure is completed.

According to another embodiment, there is provided a UE for requestingsystem information in a wireless communication system. The UE mayinclude: a memory; a transceiver; and a processor to connect the memorywith the transceiver, wherein the processor may: control the transceiverto transmit a random access preamble for requesting system informationto a BS; controls the transceiver to receive, from the BS, a randomaccess response including only a RAPID corresponding to the transmittedrandom access preamble; and considers that a random access procedure iscompleted.

A UE can efficiently request other system information.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows LTE system architecture.

FIG. 2 shows a control plane of a radio interface protocol of an LTEsystem.

FIG. 3 shows a user plane of a radio interface protocol of an LTEsystem.

FIG. 4 shows an example of transmitting a master information block(MIB), system information block1 (SIB1), and other SIBs.

FIG. 5 shows an update of system information.

FIG. 6 illustrates a contention-based random access procedure.

FIG. 7 illustrates a non-contention random access procedure.

FIG. 8 shows a procedure for a UE to receive new-type systeminformation.

FIG. 9 shows a procedure in which a UE requests system information in arandom access procedure according to an embodiment of the presentinvention.

FIG. 10 shows an example of a MAC subheader including only a RAPIDaccording to an embodiment of the present invention.

FIG. 11 shows an example of a MAC PUD according to an embodiment of thepresent invention.

FIG. 12 shows a method for a UE to request and receive systeminformation on the basis of a new type of a RAR window in a randomaccess procedure according to an embodiment of the present invention.

FIG. 13 shows an example in which requested system information isprovided in a second RAR window according to an embodiment of thepresent invention.

FIG. 14 is a block diagram illustrating a method for a UE to requestsystem information according to an embodiment of the present invention.

FIG. 15 is a block diagram illustrating a wireless communication systemaccording to the embodiment of the present invention.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

The technology described below can be used in various wirelesscommunication systems such as code division multiple access (CDMA),frequency division multiple access (FDMA), time division multiple access(TDMA), orthogonal frequency division multiple access (OFDMA), singlecarrier frequency division multiple access (SC-FDMA), etc. The CDMA canbe implemented with a radio technology such as universal terrestrialradio access (UTRA) or CDMA-2000. The TDMA can be implemented with aradio technology such as global system for mobile communications(GSM)/general packet ratio service (GPRS)/enhanced data rate for GSMevolution (EDGE). The OFDMA can be implemented with a radio technologysuch as institute of electrical and electronics engineers (IEEE) 802.11(Wi-Fi), IEEE 802.16 (WiMAX), IEEE 802.20, evolved UTRA (E-UTRA), etc.IEEE 802.16m is evolved from IEEE 802.16e, and provides backwardcompatibility with a system based on the IEEE 802.16e. The UTRA is apart of a universal mobile telecommunication system (UMTS). 3rdgeneration partnership project (3GPP) long term evolution (LTE) is apart of an evolved UMTS (E-UMTS) using the E-UTRA. The 3GPP LTE uses the01-DMA in a downlink and uses the SC-FDMA in an uplink. LTE-advanced(LTE-A) is an evolution of the LTE. 5G is an evolution of the LTE-A.

For clarity, the following description will focus on LTE-A/5G. However,technical features of the present invention are not limited thereto.

FIG. 1 shows LTE system architecture. The communication network iswidely deployed to provide a variety of communication services such asvoice over internet protocol (VoIP) through IMS and packet data.

Referring to FIG. 1, the LTE system architecture includes one or moreuser equipment (UE; 10), an evolved-UMTS terrestrial radio accessnetwork (E-UTRAN) and an evolved packet core (EPC). The UE 10 refers toa communication equipment carried by a user. The UE 10 may be fixed ormobile, and may be referred to as another terminology, such as a mobilestation (MS), a user terminal (UT), a subscriber station (SS), awireless device, etc.

The E-UTRAN includes one or more evolved node-B (eNB) 20, and aplurality of UEs may be located in one cell. The eNB 20 provides an endpoint of a control plane and a user plane to the UE 10. The eNB 20 isgenerally a fixed station that communicates with the UE 10 and may bereferred to as another terminology, such as a base station (BS), a basetransceiver system (BTS), an access point, etc. One eNB 20 may bedeployed per cell. There are one or more cells within the coverage ofthe eNB 20. A single cell is configured to have one of bandwidthsselected from 1.25, 2.5, 5, 10, and 20 MHz, etc., and provides downlinkor uplink transmission services to several UEs. In this case, differentcells can be configured to provide different bandwidths.

Hereinafter, a downlink (DL) denotes communication from the eNB 20 tothe UE 10, and an uplink (UL) denotes communication from the UE 10 tothe eNB 20. In the DL, a transmitter may be a part of the eNB 20, and areceiver may be a part of the UE 10. In the UL, the transmitter may be apart of the UE 10, and the receiver may be a part of the eNB 20.

The EPC includes a mobility management entity (MME) which is in chargeof control plane functions, and a system architecture evolution (SAE)gateway (S-GW) which is in charge of user plane functions. The MME/S-GW30 may be positioned at the end of the network and connected to anexternal network. The MME has UE access information or UE capabilityinformation, and such information may be primarily used in UE mobilitymanagement. The S-GW is a gateway of which an endpoint is an E-UTRAN.The MME/S-GW 30 provides an end point of a session and mobilitymanagement function for the UE 10. The EPC may further include a packetdata network (PDN) gateway (PDN-GW). The PDN-GW is a gateway of which anendpoint is a PDN.

The MME provides various functions including non-access stratum (NAS)signaling to eNBs 20, NAS signaling security, access stratum (AS)security control, Inter core network (CN) node signaling for mobilitybetween 3GPP access networks, idle mode UE reachability (includingcontrol and execution of paging retransmission), tracking area listmanagement (for UE in idle and active mode), P-GW and S-GW selection,MME selection for handovers with MME change, serving GPRS support node(SGSN) selection for handovers to 2G or 3G 3GPP access networks,roaming, authentication, bearer management functions including dedicatedbearer establishment, support for public warning system (PWS) (whichincludes earthquake and tsunami warning system (ETWS) and commercialmobile alert system (CMAS)) message transmission. The S-GW host providesassorted functions including per-user based packet filtering (by e.g.,deep packet inspection), lawful interception, UE Internet protocol (IP)address allocation, transport level packet marking in the DL, UL and DLservice level charging, gating and rate enforcement, DL rate enforcementbased on APN-AMBR. For clarity MME/S-GW 30 will be referred to hereinsimply as a “gateway,” but it is understood that this entity includesboth the MME and S-GW.

Interfaces for transmitting user traffic or control traffic may be used.The UE 10 and the eNB 20 are connected by means of a Uu interface. TheeNBs 20 are interconnected by means of an X2 interface. Neighboring eNBsmay have a meshed network structure that has the X2 interface. The eNBs20 are connected to the EPC by means of an S1 interface. The eNBs 20 areconnected to the MME by means of an S1-MME interface, and are connectedto the S-GW by means of S1-U interface. The S1 interface supports amany-to-many relation between the eNB 20 and the MME/S-GW.

The eNB 20 may perform functions of selection for gateway 30, routingtoward the gateway 30 during a radio resource control (RRC) activation,scheduling and transmitting of paging messages, scheduling andtransmitting of broadcast channel (BCH) information, dynamic allocationof resources to the UEs 10 in both UL and DL, configuration andprovisioning of eNB measurements, radio bearer control, radio admissioncontrol (RAC), and connection mobility control in LTE_ACTIVE state. Inthe EPC, and as noted above, gateway 30 may perform functions of pagingorigination, LTE_IDLE state management, ciphering of the user plane, SAEbearer control, and ciphering and integrity protection of NAS signaling.

FIG. 2 shows a control plane of a radio interface protocol of an LTEsystem. FIG. 3 shows a user plane of a radio interface protocol of anLTE system.

Layers of a radio interface protocol between the UE and the E-UTRAN maybe classified into a first layer (L1), a second layer (L2), and a thirdlayer (L3) based on the lower three layers of the open systeminterconnection (OSI) model that is well-known in the communicationsystem. The radio interface protocol between the UE and the E-UTRAN maybe horizontally divided into a physical layer, a data link layer, and anetwork layer, and may be vertically divided into a control plane(C-plane) which is a protocol stack for control signal transmission anda user plane (U-plane) which is a protocol stack for data informationtransmission. The layers of the radio interface protocol exist in pairsat the UE and the E-UTRAN, and are in charge of data transmission of theUu interface.

A physical (PHY) layer belongs to the L1. The PHY layer provides ahigher layer with an information transfer service through a physicalchannel. The PHY layer is connected to a medium access control (MAC)layer, which is a higher layer of the PHY layer, through a transportchannel. A physical channel is mapped to the transport channel Data istransferred between the MAC layer and the PHY layer through thetransport channel Between different PHY layers, i.e., a PHY layer of atransmitter and a PHY layer of a receiver, data is transferred throughthe physical channel using radio resources. The physical channel ismodulated using an orthogonal frequency division multiplexing (OFDM)scheme, and utilizes time and frequency as a radio resource.

The PHY layer uses several physical control channels. A physicaldownlink control channel (PDCCH) reports to a UE about resourceallocation of a paging channel (PCH) and a downlink shared channel(DL-SCH), and hybrid automatic repeat request (HARQ) information relatedto the DL-SCH. The PDCCH may carry a UL grant for reporting to the UEabout resource allocation of UL transmission. A physical control formatindicator channel (PCFICH) reports the number of OFDM symbols used forPDCCHs to the UE, and is transmitted in every subframe. A physicalhybrid ARQ indicator channel (PHICH) carries an HARQ acknowledgement(ACK)/non-acknowledgement (NACK) signal in response to UL transmission.A physical uplink control channel (PUCCH) carries UL control informationsuch as HARQ ACK/NACK for DL transmission, scheduling request, and CQI.A physical uplink shared channel (PUSCH) carries a UL-uplink sharedchannel (SCH).

A physical channel consists of a plurality of subframes in time domainand a plurality of subcarriers in frequency domain. One subframeconsists of a plurality of symbols in the time domain. One subframeconsists of a plurality of resource blocks (RBs). One RB consists of aplurality of symbols and a plurality of subcarriers. In addition, eachsubframe may use specific subcarriers of specific symbols of acorresponding subframe for a PDCCH. For example, a first symbol of thesubframe may be used for the PDCCH. The PDCCH carries dynamic allocatedresources, such as a physical resource block (PRB) and modulation andcoding scheme (MCS). A transmission time interval (TTI) which is a unittime for data transmission may be equal to a length of one subframe. Thelength of one subframe may be 1 ms.

The transport channel is classified into a common transport channel anda dedicated transport channel according to whether the channel is sharedor not. A DL transport channel for transmitting data from the network tothe UE includes a broadcast channel (BCH) for transmitting systeminformation, a paging channel (PCH) for transmitting a paging message, aDL-SCH for transmitting user traffic or control signals, etc. The DL-SCHsupports HARQ, dynamic link adaptation by varying the modulation, codingand transmit power, and both dynamic and semi-static resourceallocation. The DL-SCH also may enable broadcast in the entire cell andthe use of beamforming. The system information carries one or moresystem information blocks. All system information blocks may betransmitted with the same periodicity. Traffic or control signals of amultimedia broadcast/multicast service (MBMS) may be transmitted throughthe DL-SCH or a multicast channel (MCH).

A UL transport channel for transmitting data from the UE to the networkincludes a random access channel (RACH) for transmitting an initialcontrol message, a UL-SCH for transmitting user traffic or controlsignals, etc. The UL-SCH supports HARQ and dynamic link adaptation byvarying the transmit power and potentially modulation and coding. TheUL-SCH also may enable the use of beamforming. The RACH is normally usedfor initial access to a cell.

A MAC layer belongs to the L2. The MAC layer provides services to aradio link control (RLC) layer, which is a higher layer of the MAClayer, via a logical channel. The MAC layer provides a function ofmapping multiple logical channels to multiple transport channels. TheMAC layer also provides a function of logical channel multiplexing bymapping multiple logical channels to a single transport channel A MACsublayer provides data transfer services on logical channels.

The logical channels are classified into control channels fortransferring control plane information and traffic channels fortransferring user plane information, according to a type of transmittedinformation. That is, a set of logical channel types is defined fordifferent data transfer services offered by the MAC layer. The logicalchannels are located above the transport channel, and are mapped to thetransport channels.

The control channels are used for transfer of control plane informationonly. The control channels provided by the MAC layer include a broadcastcontrol channel (BCCH), a paging control channel (PCCH), a commoncontrol channel (CCCH), a multicast control channel (MCCH) and adedicated control channel (DCCH). The BCCH is a downlink channel forbroadcasting system control information. The PCCH is a downlink channelthat transfers paging information and is used when the network does notknow the location cell of a UE. The CCCH is used by UEs having no RRCconnection with the network. The MCCH is a point-to-multipoint downlinkchannel used for transmitting MBMS control information from the networkto a UE. The DCCH is a point-to-point bi-directional channel used by UEshaving a RRC connection that transmits dedicated control informationbetween a UE and the network.

Traffic channels are used for the transfer of user plane informationonly. The traffic channels provided by the MAC layer include a dedicatedtraffic channel (DTCH) and a multicast traffic channel (MTCH). The DTCHis a point-to-point channel, dedicated to one UE for the transfer ofuser information and can exist in both uplink and downlink. The MTCH isa point-to-multipoint downlink channel for transmitting traffic datafrom the network to the UE.

Uplink connections between logical channels and transport channelsinclude the DCCH that can be mapped to the UL-SCH, the DTCH that can bemapped to the UL-SCH and the CCCH that can be mapped to the UL-SCH.Downlink connections between logical channels and transport channelsinclude the BCCH that can be mapped to the BCH or DL-SCH, the PCCH thatcan be mapped to the PCH, the DCCH that can be mapped to the DL-SCH, andthe DTCH that can be mapped to the DL-SCH, the MCCH that can be mappedto the MCH, and the MTCH that can be mapped to the MCH.

An RLC layer belongs to the L2. The RLC layer provides a function ofadjusting a size of data, so as to be suitable for a lower layer totransmit the data, by concatenating and segmenting the data receivedfrom an upper layer in a radio section. In addition, to ensure a varietyof quality of service (QoS) required by a radio bearer (RB), the RLClayer provides three operation modes, i.e., a transparent mode (TM), anunacknowledged mode (UM), and an acknowledged mode (AM). The AM RLCprovides a retransmission function through an automatic repeat request(ARQ) for reliable data transmission. Meanwhile, a function of the RLClayer may be implemented with a functional block inside the MAC layer.In this case, the RLC layer may not exist.

A packet data convergence protocol (PDCP) layer belongs to the L2. ThePDCP layer provides a function of header compression function thatreduces unnecessary control information such that data being transmittedby employing IP packets, such as IPv4 or IPv6, can be efficientlytransmitted over a radio interface that has a relatively smallbandwidth. The header compression increases transmission efficiency inthe radio section by transmitting only necessary information in a headerof the data. In addition, the PDCP layer provides a function ofsecurity. The function of security includes ciphering which preventsinspection of third parties, and integrity protection which preventsdata manipulation of third parties.

A radio resource control (RRC) layer belongs to the L3. The RLC layer islocated at the lowest portion of the L3, and is only defined in thecontrol plane. The RRC layer takes a role of controlling a radioresource between the UE and the network. For this, the UE and thenetwork exchange an RRC message through the RRC layer. The RRC layercontrols logical channels, transport channels, and physical channels inrelation to the configuration, reconfiguration, and release of RBs. AnRB is a logical path provided by the L1 and L2 for data delivery betweenthe UE and the network. That is, the RB signifies a service provided theL2 for data transmission between the UE and E-UTRAN. The configurationof the RB implies a process for specifying a radio protocol layer andchannel properties to provide a particular service and for determiningrespective detailed parameters and operations. The RB is classified intotwo types, i.e., a signaling RB (SRB) and a data RB (DRB). The SRB isused as a path for transmitting an RRC message in the control plane. TheDRB is used as a path for transmitting user data in the user plane.

A Non-Access Stratum (NAS) layer placed over the RRC layer performsfunctions, such as session management and mobility management.

Referring to FIG. 2, the RLC and MAC layers (terminated in the eNB onthe network side) may perform functions such as scheduling, automaticrepeat request (ARQ), and hybrid automatic repeat request (HARQ). TheRRC layer (terminated in the eNB on the network side) may performfunctions such as broadcasting, paging, RRC connection management, RBcontrol, mobility functions, and UE measurement reporting andcontrolling. The NAS control protocol (terminated in the MME of gatewayon the network side) may perform functions such as a SAE bearermanagement, authentication, LTE_IDLE mobility handling, pagingorigination in LTE_IDLE, and security control for the signaling betweenthe gateway and UE.

Referring to FIG. 3, the RLC and MAC layers (terminated in the eNB onthe network side) may perform the same functions for the control plane.The PDCP layer (terminated in the eNB on the network side) may performthe user plane functions such as header compression, integrityprotection, and ciphering.

Hereinafter, system information will be described.

FIG. 4 shows an example of transmitting a master information block(MIB), system information block1 (SIB1), and other SIBs.

An LTE cell broadcasts basic parameters necessary for the operation ofan IDLE_MODE UE and a CONNECTED_MODE UE via a plurality of separateinformation blocks. Examples of information blocks include an MIB, SIB1,SIB2, and other SIBs (SIBn).

The MIB includes the most essential parameters needed for a UE to accessa cell. Referring to FIG. 4, an MIB message is broadcast through a BCHaccording to a periodicity of 40 ms, and MIB transmission is repeated inall radio frames within the periodicity of 40 ms. The UE receives an SIBmessage using the parameters received via the MIB.

There are different types of SIBs.

SIB1 includes pieces of information associated with cell access, andparticularly includes scheduling information on other SIBs (SIB2 toSIBn) than SIB1. SIBs having the same transmission periodicity among theSIBs other than SIB1 are transferred via the same system information(SI) message. Thus, scheduling information includes a mappingrelationship between each SIB and an SI message. An SI message istransmitted within an SI window in a time domain, and each SI message isassociated with one SI window. Since SI windows for different pieces ofSI do not overlap, only one SI message is transmitted within an SIwindow. Thus, scheduling information includes the duration of an SIwindow and an SI transmission periodicity. Time/frequency fortransmitting an SI message is determined by dynamic scheduling by a BS.SIB1 is broadcast through a downlink shared channel (DL SCH) accordingto a periodicity of eight radio frames (that is, 80-ms periodicity), andSIB1 is repeatedly retransmitted on a fifth subframe of an SFN-mod-2radio frame within the 80-ms periodicity.

SIB2 includes necessary information for a UE to access a cell. SIB2includes information on an uplink cell bandwidth, a random accessparameter, and an uplink power control parameter.

SIB3 includes cell reselection information. SIB4 includes frequencyinformation on a serving cell and intra-frequency information on aneighboring cell for cell reselection. SIB5 includes frequencyinformation on a different E-UTRA and inter-frequency information on aneighboring cell for cell reselection. SIB6 includes frequencyinformation on a UTRA and information on a UTRA neighboring cell forcell reselection. SIB7 includes frequency information on a GERAN forcell reselection. SIB8 includes information on a neighboring cell.

SIB9 includes a Home eNodeB (HeNB) identifier (ID). SIB10 to SIB12include a public warning message, for example, for earthquake warning.SIB14 is used to support enhanced access barring and controls UEs toaccess a cell. SIB15 includes information needed to receive an MBMS atcontiguous carrier frequencies. SIB16 include GPS time and coordinateduniversal time (UTC)-related information. SIB17 includes RAN auxiliaryinformation.

Not all SIBs are always required to be present. For example, SIB9 is notneeded in a mode where a wireless carrier establishes an HeNB, whileSIB13 is not needed if a cell provides no MBMS.

System information is commonly applied to all UEs accessing a cell, andUEs need to always maintain up-to-date system information to perform anappropriate operation.

When system information is changed, UEs need to know in advance the timethe BS transmits new system information. In order that a BS and a UEmutually recognize a radio frame period for transmitting new systeminformation, the concept of BCCH modification period is introduced in“3GPP TS 36.331 v9.3.0,” which is described in detail.

FIG. 5 shows an update of system information.

Referring to FIG. 5, a BS, which intends to update system information inan (n+1)th modification period, notifies in advance UEs of an update ofsystem information in an nth modification period. A UE, which isnotified the update of the system information in the nth modificationperiod, receives and applies new system information at the verybeginning of the (n+1)th modification period. When an update of systeminformation is scheduled, the BS includes a system informationmodification indicator in a paging message. Generally, a paging messageis a message received by an idle-mode UE. However, since an update ofsystem information is notified through a paging message, aconnected-mode UE also needs to receive a paging message at times and toidentify an update of system information.

Hereinafter, random access will be described.

Random access is used by a UE to obtain uplink synchronization with a BSor to be allocated an uplink radio resource. After power is turned on, aUE obtains downlink synchronization with an initial cell and receivessystem information. Then, the UE acquires, from the system information,a set of available random access preambles and information about a radioresource used for transmission of a random access preamble. The radioresource used for transmission of the random access preamble may bespecified as a radio frame and/or a combination of at least one or moresubframes. The UE transmits a random access preamble randomly selectedfrom the set of random access preambles, and the BS having received therandom access preamble sends a timing alignment (TA) value for uplinksynchronization to the UE through a random access response. Thus, the UEobtains uplink synchronization.

That is, the BS allocates a dedicated random access preamble to aspecific UE, and the UE performs non-contention random access using therandom access preamble. That is, there may be in a process of selectinga random access preamble, contention-based random access in which a UErandomly selects and uses one random access preamble from a particularset and non-contention random access in which only a specific UE isallocated a random access preamble by a BS. Non-contention random accessmay be used for a handover procedure or upon a request by a BS'scommand.

FIG. 6 illustrates a contention-based random access procedure.

Referring to FIG. 6, a UE randomly selects one random access preamblefrom a random access preamble set indicated by system information or ahandover command. The UE selects a radio resource for transmitting therandom access preamble to transmit the selected random access preamble(S610). The radio resource may be a specific subframe, and selecting theradio resource may be selecting a physical random access channel(PRACH).

After transmitting the random access preamble, the UE attempts toreceive a random access response within a random access responsereception window indicated by the system information or the handovercommand and accordingly receives a random access response (S620). Therandom access response may be transmitted in an MAC PDU format, and theMAC PDU may be forwarded via a physical downlink shared channel (PDSCH).Further, a physical downlink control channel (PDCCH) is also forwardedso that the UE properly receives information forwarded via the PDSCH.That is, the PDCCH includes information on the UE receiving the PDSCH,frequency and time information on a radio resource for the PDSCH, and atransmission format for the PDSCH. Once successfully receiving the PDCCHforwarded to the UE, the UE properly receives the random access responsetransmitted via the PDSCH on the basis of the information in the PDCCH.

The random access response may include a random access preambleidentifier (ID), an uplink radio resource (UL grant), a temporarycell-radio network temporary identifier (C-RNTI), and a time alignmentcommand (TAC). Since one random access response may include randomaccess response information for one or more UEs, a random accesspreamble ID may be included to indicate a UE for which a UL grant, atemporary C-RNTI, and a TAC are valid. The random access preamble ID maybe an ID of the random access preamble received by a BS. The TAC may beincluded as information for the UE to adjust uplink synchronization. Therandom access response may be indicated by a random access ID on thePDCCH, that is, a random access-radio network temporary identifier(RA-RNTI).

When the UE receives the random access response valid therefor, the UEprocesses information included in the random access response andperforms scheduled transmission to the BS (S630). That is, the UEapplies the TAC and stores the temporary C-RNTI. Further, the UEtransmits data stored in a buffer of the UE or newly generated data tothe BS using the UL grant. In this case, information to identify the UEneeds to be included, which is for identifying the UE in order to avoida collision since the BS does not determine which UEs perform randomaccess in a contention-based random access process.

There are two methods for including information for identifying a UE.When the UE has a valid cell ID already allocated by a correspondingcell before performing random access, the UE transmits the cell IDthereof through the UL grant. However, when the UE is not allocated avalid cell ID before the random access process, the UE transmits aunique ID thereof (e.g, S-TMSI or random ID). Generally, the unique IDis longer than the cell ID. When the UE transmits the data via the ULgrant, the UE starts a contention resolution timer.

After transmitting the data including the ID of the UE through the ULgrant allocated by receiving the random access response, the UE waitsfor an instruction from the BS to avoid a collision (S640). That is, theUE attempts to receive the PDCCH in order to receive a specific message.There are two proposed methods for receiving a PDCCH. As describedabove, when the ID of the UE transmitted via the UL grant is a cell ID,the UE may attempt to receive the PDCCH using the cell ID of the UE. Inthis case, when the UE receives the PDCCH through the cell ID of the UEbefore the contention resolution timer expires, the UE determines thatrandom access has been normally performed and terminates random access.When the ID transmitted via the UL grant is the unique ID, the UE mayattempt to receive the PDCCH using the temporary C-RNTI included in therandom access response. In this case, when the UE receives the PDCCHthrough the temporary cell ID before the contention resolution timerexpires, the UE identifies data forwarded by the PDSCH indicated by thePDCCH. When the data includes the unique ID of the UE, the UE maydetermine that random access has been normally performed and mayterminate random access.

FIG. 7 illustrates a non-contention random access procedure.

Unlike contention-based random access, non-contention random access maybe terminated when a UE receives a random access response.

Non-contention random access may be initiated by a request, such as ahandover and/or a command from a BS. Here, in these two cases,contention-based random access may also be performed.

The UE is allocated by the BS a designated random access preamble havingno possibility of a collision. The random access preamble may beallocated through a handover command and a PDCCH command (S710).

After being allocated the random access preamble designated for the UE,the UE transmits the random access preamble to the BS (S720).

Upon receiving the random access preamble, the BS transmits a randomaccess response to the UE in response (S730). A procedure associatedwith the random access response has been mentioned above in S620 of FIG.6.

The number of system information blocks is continuously increasing, andradio resources are required to broadcast a system information block.Thus, as the number of system information blocks increases, the quantityof radio resources required to broadcast a system information block alsoinevitably increases. To solve such a problem, new-type systeminformation is proposed.

FIG. 8 shows a procedure for a UE to receive new-type systeminformation.

Referring to FIG. 8, the new-type system information may be divided intominimum system information and other system information. The minimumsystem information may be periodically broadcasted. The minimum systeminformation may include basic information required for initial access toa cell and information for acquiring any other system information thatis provisioned on an on-demand basis or is periodically broadcasted. Theminimum system information may include at least one of a SFN, a list ofPLMNs, a cell ID, a cell camping parameter, and a RACH parameter. When anetwork allows an on-demand mechanism, a parameter required to requestthe other system information may be included in the minimum systeminformation. The other system information may refer to all systeminformation not broadcast in the minimum system information.

Meanwhile, a UE may request a network to transmit system information inorder to acquire other system information. For example, when the networkdoes not broadcast specific system information, the UE in the RRC_IDLEmode may request the specific system information from the network usinga RACH procedure. When the UE requests the specific system informationfrom the network using a RACH procedure, a first message may be used torequest system information, and the requested system information may bebroadcast. When the first message is used to request the systeminformation, the UE may not need to transmit a third message to thenetwork. Furthermore, when the first message is used to request thesystem information, the UE does not need to transmit the third messageto the network, and thus a UL grant for the third message does not needto be included in a second message. Hereinafter, a method for a UE torequest system information in a random access procedure and a devicesupporting the system information will be described according to anembodiment of the present invention. In the present specification, arandom access procedure for requesting system information may also bereferred to as a system information request procedure. In the presentspecification, a message transmitted first in a random access proceduremay be referred to as a first message or MSG1, a message transmittedsecond may be referred to as a second message or MSG2, a messagetransmitted third may be referred to as a third message or MSG3, and amessage transmitted fourth may be referred to as a fourth message orMSG4.

FIG. 9 shows a procedure in which a UE requests system information in arandom access procedure according to an embodiment of the presentinvention.

Referring to FIG. 9, in step S910, a UE may transmit a first message toa BS. The first message may be a random access preamble. The randomaccess preamble may be used to request system information. The firstmessage may be transmitted using a first message resource reserved torequest system information. For example, when the UE desires to receiveother system information, the UE may select a first message resourcecorresponding to other system information of interest and may transmit afirst message requesting transmission of the system information usingthe selected first message resource. The UE may be in an RRC_IDLE stateor an RRC_INACTIVE state.

In step S920, the UE may receive, from the BS, a second messageincluding a random access preamble identifier (RAPID) corresponding tothe transmitted random access preamble. That is, the UE may receive,from the BS, a second message including a first resource identifier thatmatches the transmitted first message resource. The second message maybe a random access response or a system information request response.

The second message may include only the RAPID. When the random accesspreamble for requesting the system information is transmitted, the BSmay transmit, to the UE, the second message including only the RAPIDcorresponding to the transmitted random access preamble. The secondmessage may include only the RAPID corresponding to the random accesspreamble transmitted to request the system information in step S910 butmay not include a medium access control random access response (MACRAR). That is, the second message may not include a UL grant mapped tothe random access preamble transmitted to request the system informationin step S910. When the RAPID corresponds to any one of random accesspreambles set to request the system information, a MAC RAR may not beincluded in a MAC sub-PDU.

FIG. 10 shows an example of a MAC subheader including only a RAPIDaccording to an embodiment of the present invention.

Referring back to FIG. 9, in step S930, when the UE receives the secondmessage including only the RAPID (i.e., not including a MAC RAR or a ULgrant), the UE may determine that the random access procedure forrequesting the system information is completed. Accordingly, the UE mayterminate the random access procedure for requesting the systeminformation. Therefore, the UE may not transmit a third message to theBS. The UE may expect that the requested system information will bebroadcast. In addition, the UE may report to a higher layer that an ACKof the request for the system information is received.

In step S940, the UE may verify when the requested system informationwill be broadcast and may receive the requested system information. Therequested system information may be received in a broadcast manner.

Alternatively, although not shown in FIG. 9, in step S920, the UE mayreceive a second message including a MAC RAR corresponding to thetransmitted random access preamble. Accordingly, upon receiving thesecond message including a UL grant, the UE may perform a four-steprandom access procedure and may enter the RRC_CONNECTED state. That is,the UE may transmit a third message to the BS, may receive a fourthmessage from the BS, and may enter the RRC_CONNECTED state. Then, the UEmay receive the requested system information in a dedicated manner.

According to the embodiment of the present invention, when the UEtransmits a random access preamble for requesting system information tothe BS, the BS may transmit a random access response including only aRAPID corresponding to the transmitted random access preamble to the UE.Upon receiving the random access response, the UE may determine that arandom access procedure for requesting the system information iscompleted. Accordingly, it is possible to prevent the waste of radioresources or battery consumption which may occur when the UEunnecessarily transmits a third message to the BS.

FIG. 11 shows an example of a MAC PUD according to an embodiment of thepresent invention.

Referring to FIG. 11, the MAC PDU may include a MAC PDU header and zeroor more MAC RARs. One MAC PDU header may include one or more MAC PDUsubheaders. For each MAC PDU subheader including a RAPID, acorresponding MAC RAR may or may not be included in the MAC PDU. A firstMAC subheader including a RAPID may be mapped to a first MAC RAR. Asecond MAC subheader including a RAPID may be mapped to a second MACRAR. That is, the MAC subheader including RAPID 2 may be mapped to thefirst MAC RAR including a UL grant, and the MAC subheader includingRAPID 4 may be mapped to the second MAC RAR including a UL grant.However, third and fourth MAC subheaders including a RAPID may not bemapped to any MAC RARs.

In the embodiment of FIG. 11, when a UE has used a first messageresource having RAPID 2 or RAPID 4, the UE may perform a four-steprandom access procedure. That is, since the UE has received a randomaccess response including a UL grant in response to a random accesspreamble, the UE may transmit a third message and may receive a fourthmessage after receiving a second message.

In the embodiment of FIG. 11, when the UE has used a first messageresource having RAPID 1 or RAPID 3, the UE may determine that systeminformation has been successfully requested. Thus, the UE may nottransmit a third message to complete a random access procedure. Sincethe UE has received a random access response not including a UL grant inresponse to a random access preamble, the UE may complete the randomaccess procedure without transmitting the third message.

In addition, in the embodiment of FIG. 11, a new indication including aRAPID may be included in a MAC subheader to indicate whether a MAC RARis included in the MAC PDU.

Hereinafter, a method for a UE to request and receive system informationon the basis of a new type of a RAR window in a random access procedureand a device supporting the same will be described according to anembodiment of the present invention. A network having received a firstmessage may need to determine whether to broadcast or unicast systeminformation requested by a UE and may require more time therefor. Thus,when the first message is used for requesting the system information, aconventional RAR may not be suitable. Therefore, it may be necessary topropose a new type of a RAR window. In the present specification, afirst RAR window may be a RAR window used when a first message istransmitted for a general RACH purpose, and a second RAR window may be aRAR window used when the first message is transmitted for the purpose ofrequesting system information. When the first message is transmitted forthe general RACH purpose, rather than for the purpose of requestingsystem information, a second message may be received within the firstRAR window. However, when the first message is transmitted for thepurpose of requesting system information, the second message may bereceived in the second RAR window. For example, when a UE transmits thefirst message using a resource reserved for requesting systeminformation, the UE may apply a configuration for the second RAR windowto receive the second message from the network. Otherwise, the UE mayapply a configuration for the first RAR window to receive the secondmessage from the network.

FIG. 12 shows a method for a UE to request and receive systeminformation on the basis of a new type of a RAR window in a randomaccess procedure according to an embodiment of the present invention.Specifically, (a) of FIG. 12 shows an example in which a first messageis transmitted for a general RACH purpose, and (b) and (c) of FIG. 12show an example in which a first message is transmitted for the purposeof requesting system information.

Referring to (a) of FIG. 12, in step S1201, the UE may initiate a RACHprocedure to establish a RRC connection. The UE may select a firstmessage resource and may transmit a first message using the selectedfirst message resource. The first message may be a random accesspreamble. The selected first message resource is not a resourceassociated with a request for system information. Thus, the UE mayexpect that a second message will be received within a first RAR window.The second message may be a random access response.

In step S1202, the UE may receive the second message in the first RARwindow. The second message may be received according to a first RARconfiguration. In step S1203, the UE may transmit a third message to anetwork. The third message may include a UE ID. In step S1204, the UEmay receive a fourth message from the network. For example, the fourthmessage may be a RRC connection setup message. Then, the UE may enterthe RRC_CONNECTED state.

Referring to (b) of FIG. 12, in step S1211, when a UE desires to receiveother system information, the UE may select a first message resourcecorresponding to other system information of interest. The UE maytransmit a first message requesting transmission of the systeminformation using the selected first message resource. The first messagemay be a random access preamble. The selected first message resource isa resource associated with the request for the system information. Thus,the UE may expect that a second message will be received in a second RARwindow. The second message may be a random access response or a systeminformation request response.

Additionally, a network may determine whether to broadcast or unicastthe requested system information. In (b) of FIG. 12, it is assumed thatthe network determines to broadcast the requested system information.

In step S1212, the UE may receive the second message including a RAPIDcorresponding to the transmitted random access preamble in the secondRAR window. The second message may be received according to a second RARconfiguration. The second RAR configuration may be periodicallybroadcast along with a first RAR configuration. When the second messageincluding the RAPID corresponding to the transmitted random accesspreamble is received, the UE may determine that the system informationhas been successfully requested. Otherwise, the UE may consider that therequest for the system information has failed and may retransmit thefirst message requesting the system information.

The second message may not include a UL grant or MAC RAR mapped to thetransmitted random access preamble. When the UE receives the secondmessage that does not include the UL grant or MAC RAR mapped to thetransmitted random access preamble, the UE may consider that a RACHprocedure for requesting the system information or a system informationrequest procedure is completed. The UE may stop or complete the RACHprocedure for requesting the system information or the systeminformation request procedure. Additionally, the UE may expect that therequested system information will be broadcast.

In step S1213, the UE may check when the requested system information isbroadcast. The UE may receive the requested system information in abroadcast manner.

Referring to (c) of FIG. 12, in step S1221, when a UE desires to receiveother system information, the UE may select a first message resourcecorresponding to other system information of interest. The UE maytransmit a first message requesting transmission of the systeminformation using the selected first message resource. The first messagemay be a random access preamble. The selected first message resource isa resource associated with the request for the system information. Thus,the UE may expect that a second message will be received in a second RARwindow. The second message may be a random access response or a systeminformation request response.

Additionally, a network may determine whether to broadcast or unicastthe requested system information. In (c) of FIG. 12, it is assumed thatthe network determines to unicast the requested system information.

In step S1222, the UE may receive the second message including a RAPIDcorresponding to the transmitted random access preamble in the secondRAR window. The second message may be received according to a second RARconfiguration. The second RAR configuration may be periodicallybroadcast along with a first RAR configuration. When the second messageincluding the RAPID corresponding to the transmitted random accesspreamble is received, the UE may determine that the system informationhas been successfully requested. Otherwise, the UE may consider that therequest for the system information has failed and may retransmit thefirst message requesting the system information.

The second message may include a UL grant or MAC RAR mapped to thetransmitted random access preamble. When the UE receives the secondmessage that includes the UL grant or MAC RAR mapped to the transmittedrandom access preamble, the UE may continue a RACH procedure forrequesting the system information or a system information requestprocedure. The UE may expect that the requested system information willbe unicast and may continue the four-step RACH procedure to receive therequested system information in a dedicated manner.

In step S1223, the UE may transmit a third message to the network. Thethird message may include a UE ID. In step S1224, the UE may receive afourth message from the network. For example, the fourth message may bea RRC connection setup message.

In step S1225, the UE may enter the RRC_CONNECTED state and may receivethe requested system information through dedicated signaling.

FIG. 13 shows an example in which requested system information isprovided in a second RAR window according to an embodiment of thepresent invention.

Referring to (a) of FIG. 13, when a UE transmits a first message in anNth second RAR window, the UE may expect that a second message will betransmitted in an (N+1)th second RAR window. A configuration for thesecond RAR windows may be periodically broadcast.

Referring to (b) of FIG. 13, when a plurality of UEs requests a systeminformation block in an Nth second RAR window, a network may determineto broadcast the requested system information block in an (N+1)th secondRAR window. In this case, there may be no MAC RAR corresponding to a MACsubheader. However, one UE requests a system information block in theNth second RAR window, the network may determine to broadcast therequested system information block in the (N+1)th second RAR window.Alternatively, the network may determine to unicast the requested systeminformation block in the (N+1)th second RAR window. In this case, theremay be a MAC RAR including a UL grant corresponding to a MAC subheader.

FIG. 14 is a block diagram illustrating a method for a UE to requestsystem information according to an embodiment of the present invention.

Referring to FIG. 14, in step S1410, the UE may transmit a random accesspreamble for requesting system information to a BS.

In step S1420, the UE may receive, from the BS, a random access responseincluding only a RAPID corresponding to the transmitted random accesspreamble. The random access response may not include a MAC RARcorresponding to the RAPID. The random access response may not include aUL grant corresponding to the RAPID. The random access responseincluding only the RAPID may be an ACK of the request for the systeminformation. The random access response may be received from the BSusing a MAC PDU.

The random access response may be received in a RAR window newly definedto receive the random access response corresponding to the random accesspreamble for requesting the system information.

In step S1430, the UE may consider that a random access procedure iscompleted. When the UE receives the random access response includingonly the RAPID, it is considered that the random access procedure iscompleted.

In the random access procedure, a third message may not be transmittedto the BS in response to the random access response.

In addition, the UE may transmit, to a higher layer, receipt of the ACKof the request for the system information.

In addition, the UE may check that the requested system information isbroadcast. The UE may receive the requested system information.

FIG. 15 is a block diagram illustrating a wireless communication systemaccording to the embodiment of the present invention.

A BS 1500 includes a processor 1501, a memory 1502 and a transceiver1503. The memory 1502 is connected to the processor 1501, and storesvarious information for driving the processor 1501. The transceiver 1503is connected to the processor 1501, and transmits and/or receives radiosignals. The processor 1501 implements proposed functions, processesand/or methods. In the above embodiment, an operation of the basestation may be implemented by the processor 1501.

A UE 1510 includes a processor 1511, a memory 1512 and a transceiver1513. The memory 1512 is connected to the processor 1511, and storesvarious information for driving the processor 1511. The transceiver 1513is connected to the processor 1511, and transmits and/or receives radiosignals. The processor 1511 implements proposed functions, processesand/or methods. In the above embodiment, an operation of the UE may beimplemented by the processor 1511.

The processor may include an application-specific integrated circuit(ASIC), a separate chipset, a logic circuit, and/or a data processingunit. The memory may include a read-only memory (ROM), a random accessmemory (RAM), a flash memory, a memory card, a storage medium, and/orother equivalent storage devices. The transceiver may include abase-band circuit for processing a wireless signal. When the embodimentis implemented in software, the aforementioned methods can beimplemented with a module (i.e., process, function, etc.) for performingthe aforementioned functions. The module may be stored in the memory andmay be performed by the processor. The memory may be located inside oroutside the processor, and may be coupled to the processor by usingvarious well-known means.

Various methods based on the present specification have been describedby referring to drawings and reference numerals given in the drawings onthe basis of the aforementioned examples. Although each method describesmultiple steps or blocks in a specific order for convenience ofexplanation, the invention disclosed in the claims is not limited to theorder of the steps or blocks, and each step or block can be implementedin a different order, or can be performed simultaneously with othersteps or blocks. In addition, those ordinarily skilled in the art canknow that the invention is not limited to each of the steps or blocks,and at least one different step can be added or deleted withoutdeparting from the scope and spirit of the invention.

The aforementioned embodiment includes various examples. It should benoted that those ordinarily skilled in the art know that all possiblecombinations of examples cannot be explained, and also know that variouscombinations can be derived from the technique of the presentspecification. Therefore, the protection scope of the invention shouldbe determined by combining various examples described in the detailedexplanation, without departing from the scope of the following claims.

What is claimed is:
 1. A method for performing wireless communication bya user equipment (UE), the method comprising: transmitting, to a basestation, a random access preamble for system information request;receiving, from the base station, random access response (RAR) includinga medium access control (MAC) sub protocol data unit (PDU) with a randomaccess preamble identifier (RAPID), wherein the RAPID in a MAC subheaderof the MAC subPDU corresponds to the random access preamble for systeminformation request, and wherein, based on the RAPID in the MACsubheader of the MAC subPDU corresponding to the random access preamblefor system information request, a MAC RAR is not included in the MACsubPDU; determining that reception of the RAR is successful, based onthe RAR including the MAC subPDU with the RAPID corresponding to therandom access preamble; determining that a random access procedure issuccessfully completed, (i) based on determining that the reception ofthe RAR is successful, and (ii) based on the RAR including the MACsubPDU with RAPID only; and receiving, from the base station, requestedsystem information.
 2. The method of claim 1, wherein the MAC subPDUwith RAPID only does not include the MAC RAR.
 3. The method of claim 2,wherein the MAC subPDU with RAPID only includes (i) 1-bit extensionfield, (ii) 1-bit type field, and (iii) 6-bit RAPID field including theRAPID corresponding to the random access preamble for system informationrequest.
 4. The method of claim 1, wherein the requested systeminformation is received from the base station without transmitting amessage to the base station in response to the RAR.
 5. The method ofclaim 4, wherein the requested system information is received from thebase station based on the RAR including the MAC subPDU with RAPID only.6. The method of claim 1, wherein the RAR is received from the basestation through a MAC PDU.
 7. The method of claim 6, wherein the MAC PDUincludes one or more MAC subPDUs.
 8. A user equipment (UE) configured toperform wireless communication, the UE comprising: one or more memoriesstoring instructions; one or more transceivers; and one or moreprocessors connected to the one or more memories and the one or moretransceivers, wherein the one or more processors execute theinstructions to: transmit, to a base station, a random access preamblefor system information request; receive, from the base station, randomaccess response (RAR) including a medium access control (MAC) subprotocol data unit (PDU) with a random access preamble identifier(RAPID), wherein the RAPID in a MAC subheader of the MAC subPDUcorresponds to the random access preamble for system informationrequest, and wherein, based on the RAPID in the MAC subheader of the MACsubPDU corresponding to the random access preamble for systeminformation request, a MAC RAR is not included in the MAC subPDU;determine that reception of the RAR is successful, based on the RARincluding the MAC subPDU with the RAPID corresponding to the randomaccess preamble; determine that a random access procedure issuccessfully completed, (i) based on determining that the reception ofthe RAR is successful, and (ii) based on the RAR including the MACsubPDU with RAPID only; and receive, from the base station, requestedsystem information.
 9. The UE of claim 8, wherein the MAC subPDU withRAPID only does not include the MAC RAR.
 10. The UE of claim 9, whereinthe MAC subPDU with RAPID only includes (i) 1-bit extension field, (ii)1-bit type field, and (iii) 6-bit RAPID field including the RAPIDcorresponding to the random access preamble for system informationrequest.
 11. The UE of claim 8, wherein the requested system informationis received from the base station without transmitting a message to thebase station in response to the RAR.
 12. The UE of claim 11, wherein therequested system information is received from the base station based onthe RAR including the MAC subPDU with RAPID only.
 13. The UE of claim 8,wherein the RAR is received from the base station through a MAC PDU. 14.The UE of claim 13, wherein the MAC PDU includes one or more MACsubPDUs.
 15. An apparatus configured to control a user equipment (UE),the apparatus comprising: one or more processors; and one or morememories operably connected to the one or more processors and storinginstructions, wherein the one or more processors execute theinstructions to: transmit, to a base station, a random access preamblefor system information request; receive, from the base station, randomaccess response (RAR) including a medium access control (MAC) subprotocol data unit (PDU) with a random access preamble identifier(RAPID), wherein the RAPID in a MAC subheader of the MAC subPDUcorresponds to the random access preamble for system informationrequest, and wherein, based on the RAPID in the MAC subheader of the MACsubPDU corresponding to the random access preamble for systeminformation request, a MAC RAR is not included in the MAC subPDU;determine that reception of the RAR is successful, based on the RARincluding the MAC subPDU with the RAPID corresponding to the randomaccess preamble; determine that a random access procedure issuccessfully completed, (i) based on determining that the reception ofthe RAR is successful, and (ii) based on the RAR including the MACsubPDU with RAPID only; and receive, from the base station, requestedsystem information.
 16. The apparatus of claim 15, wherein the MACsubPDU with RAPID only does not include the MAC RAR.
 17. The apparatusof claim 16, wherein the MAC subPDU with RAPID only includes (i) 1-bitextension field, (ii) 1-bit type field, and (iii) 6-bit RAPID fieldincluding the RAPID corresponding to the random access preamble forsystem information request.
 18. The apparatus of claim 15, wherein therequested system information is received from the base station withouttransmitting a message to the base station in response to the RAR. 19.The apparatus of claim 18, wherein the requested system information isreceived from the base station based on the RAR including the MAC subPDUwith RAPID only.
 20. The apparatus of claim 15, wherein the RAR isreceived from the base station through a MAC PDU, and wherein the MACPDU includes one or more MAC subPDUs.